Olive mill solid waste gasification in a pilot-scale downdraft gasifier with three-stage air supply: performance, mass-energy balance and fate of inorganic elements

Olive mill solid waste (OMSW), also called olive pomace, is a biomass waste available in many Mediterranean countries, including developing countries. In this study, OMSW has been tested as a fuel in an original pilot-scale downdraft gasifier with three-stage air supply, and extensive metrology was implemented. After pelletizing and characterization of the biomass waste, gasification was investigated by operating a 2-day campaign using air as a gasifying agent. The evolution over time of operating parameters such as flowrates, temperatures and pressure drop were recorded. The syngas (including tars) and solid residues were analyzed. The fate of inorganic elements was investigated. Mass and energy balances were performed and the performance of the gasifier was evaluated. The syngas contained 14 % v/v H2 and 17 % v/v CO (dry and tar-free basis) and its lower heating value was 4.7 MJ/Nm3 (wet basis). The solid residues were rich in potassium and phosphorus and could be considered as an organic amendment. Cold gas efficiency was 68 % and carbon conversion efficiency was 89 %. The points of caution concerned the low melting point of the ash, the relatively high tar content in the syngas and the high NOx content of the flue gases after syngas combustion in a boiler. The energy potential of OMSW is substantial and could help reduce the pressure on fossil fuels.